SARLink: Satellite Backscatter Connectivity using Synthetic Aperture Radar

TL;DR

SARLink introduces a passive, ground-based backscatter system that uses existing spaceborne SAR satellites to deliver low-bandwidth data from remote locations without modifying satellite hardware. The approach hinges on a subaperture processing method to extract multiple bits from a single SAR pass by modulating a cooperative ground target's radar cross section (RCS) with a mechanically modulating corner reflector, enabling on-off keying (OOK) communications. Theoretical analysis predicts achievable bit rates of tens of bits per pass (e.g., 60 bits per pass at $E_b/N_0\approx$ $25$ dB for a $5.5\text{ft} \times 5.5\text{ft}$ reflector) with BER on the order of $1\%$, while field experiments with Sentinel-1A validate the detectability of RCS state changes and the efficacy of the data-processing pipeline. The results demonstrate the feasibility of ultra-long-range, low-power satellite backscatter and lay groundwork for practical passive IoT-like links using existing SAR infrastructure.

Abstract

SARLink is a passive satellite backscatter communication system that uses existing spaceborne synthetic aperture radar (SAR) imaging satellites to provide connectivity in remote regions. It achieves orders of magnitude more range than traditional backscatter systems, enabling communication between a passive ground node and a satellite in low earth orbit. The system is composed of a cooperative ground target, a SAR satellite, and a data processing algorithm. A mechanically modulating reflector was designed to apply amplitude modulation to ambient SAR backscatter signals by changing its radar cross section. These communication bits are extracted from the raw SAR data using an algorithm that leverages subaperture processing to detect multiple bits from a target in a single image dataset. A theoretical analysis of this communication system using on-off keying is presented, including the expected signal model, throughput, and bit error rate. The results suggest a 5.5 ft by 5.5 ft modulating corner reflector could send 60 bits every satellite pass, enough to support low bandwidth sensor data and messages. Using Sentinel-1A, a SAR satellite at an altitude of 693~km, we deployed static and modulating reflectors to evaluate the system. The results, successfully detecting the changing state of a modulating ground target, demonstrate our algorithm's effectiveness for extracting bits, paving the way for ultra-long-range, low-power satellite backscatter communication.

Figures (16)

• Figure 1: The corner reflector modulates incoming SAR signals by changing its radar cross section as the satellite images.
• Figure 2: The side view (A) shows the corner reflector's boresight aligned with the incident SAR signals by tilting it at angle $\boldsymbol{\theta_{tilt}}$ to maximize its RCS based on the incidence angle, $\boldsymbol{\theta_{i}}$. The 3D view (B) shows how when $\boldsymbol{\alpha=45\degree}$ and $\boldsymbol{\theta=35.26\degree}$ the boresight of the reflector is aligned with incident waves and redirects them back to their source with maximum power.
• Figure 3: (A) The satellite moves in the along-track or azimuth direction and images targets on the ground, whose range can be defined using either the slant or ground range geometry and are related using the incidence angle, $\boldsymbol{\theta_{i}}$. (B) SAR processing takes the raw IQ samples and convolves them in the range direction with a matched filter and then again in the azimuth direction with another matched filter. This process compresses the data into range and azimuth bins that form the final pixels of the resolved SAR image.
• Figure 4: Subaperture processing is used to enable backscatter communication in (A) each color represents a subset of data collected at different times during the satellite pass and is used to generate separate sublooks. (B) Subaperture processing uses the same range of compression processing but adapts the azimuth compression step to produce the desired number of sublooks.
• Figure 5: SARLink enables long-range backscatter communication using existing spaceborne SAR infrastructure by introducing a cooperative on-ground target as a modulating corner reflector. A standard laptop PC with access to the Internet can run the proposed data processing algorithm to extract the input data from the satellite's images.